In order to realize a large capacity of a WDM optical communication system, it is useful to increase a transmission rate per wavelength. When a symbol rate of symbols transmitted to an optical transmission line is increased without changing a modulation method, there is a problem in that wavelength dispersion tolerance of the optical transmission line reduces because an allowable residual dispersion amount is inversely proportional to the square of the symbol rate. It is necessary to execute electrical signal processing at a high rate, and hence there is a problem in that the cost of an analog electrical part increases.
Therefore, in recent years, researches for improving signal multiplicity per symbol without increasing the symbol rate so as to realize the large capacity of the system have been actively conducted.
Known examples of a method of improving the signal multiplicity include multi-level modulation methods such as a QPSK method of assigning two values (multiplicity is two) to each symbol to increase a transmission capacity two times, a 16-QAM method of assigning four values (multiplicity is four) to each symbol to increase the transmission capacity four times, and a 16-APSK method.
In general, when any of the multi-level modulation methods is executed, an I/Q modulator is used as an optical modulator. The I/Q modulator is modulator capable of independently generating orthogonal optical electric field components (I channel and Q channel) and has a special structure in which Mach-Zehnder (MZ) modulators are connected in parallel.
For example, when the QPSK modulation method is to be executed, a dual parallel MZ modulator (DPMZM) is used in which two MZ modulators are connected in parallel (see, for example, Patent Literature 1).
When the modulation by 16-QAM is executed, the DPMZM or a quad parallel MZ modulator (QPMZM) in which two DPMZMs are connected in parallel is used (see, for example, Non-Patent Literature 1).
Even when any of the modulators as described in Patent Literature 1 and Non-Patent Literature 1 is used, the number of MZ modulators increases, and hence there is a problem in that a cost and the number of bias control points increase.
Therefore, it is expected to use a dual-electrode MZ modulator (dual drive MZM (DDMZM)) in which two phase modulators are connected in parallel, so as to realize the multi-level modulation (see, for example, Patent Literature 2 and Non-patent Literature 2).
The dual-electrode MZ modulator is an optical part widely applied as a push-pull optical modulator to a normal optical transmitter-receiver, and hence a reduction in cost may be realized. In addition, a light insertion loss may be reduced because of the structure in which light passes through the MZ modulator only once.